THE PLANT AND ITS FOOD) 25 
A solution of chlorophyll examined by 
means of a spectroscope exhibits a number 
of very definite absorption bands, due to the 
absorption of certain of the coloured rays 
of the spectrum (= the dark bands) while 
the rest of the light filters through and is 
unaffected. There are two very dark bands 
in the red region of the spectrum, and others, 
mostly fainter and more diffuse, in the yellow, 
green and blue-violet regions. Furthermore 
the extreme red and the violet end of the 
spectrum are also obliterated. 
It is found, as might perhaps be anticipated, 
that the rays of light which correspond to the 
dark absorption bands in the red region of 
the spectrum are those principally concerned 
in promoting photosynthesis. The other rays 
which are absorbed are not indeed without 
influence, but they are of comparatively little 
consequence from the point of view with which 
we are just now concerned. We see from the 
foregoing why it is so essential that the chloro- 
phyll in the living plant should be directly 
exposed to the light from the sky, inasmuch 
as any light which has already traversed a 
layer of chlorophyll will have been deprived 
of those rays that are essential for photo- 
synthesis. Such “filtered” light will of 
course be unable to develop photosynthetic 
activity in a chlorophyll-containing organ or 
organism that may be exposed to it. The 
apparent exceptions afforded by plants which 
flourish in deep shade are due to the circum- 
